Apparently there's no electrical forum so I'm posting here. I have an OMC Saildrive which is essentially a detuned version of a Johnson 25/35 outboard. The boat currently has no main fuse. Does anyone know how much the starter draws?
Power Requirements
- Thread starter Project_Mayhem
- Start date
Probably a few hundred amps for a few seconds. Fuse the wire at the battery using a fuse sized to protect the wire.
Use one of these guys: https://www.defender.com/product3.j...e-block&path=-1|328|2290021|2290030&id=983740 It attaches to the battery post and the cable attaches to the fuse. Easiest and least expensive way of adding a fuse to a battery cable.
ABYC does not require the starter cable to be fused. It is still a good idea, though.
Check an Ampacity chart to see what size fuse is necessary to protect the wire.
Use one of these guys: https://www.defender.com/product3.j...e-block&path=-1|328|2290021|2290030&id=983740 It attaches to the battery post and the cable attaches to the fuse. Easiest and least expensive way of adding a fuse to a battery cable.
ABYC does not require the starter cable to be fused. It is still a good idea, though.
Check an Ampacity chart to see what size fuse is necessary to protect the wire.
Lots for a short duration.
https://marinehowto.com/universal-diesel-engine-wiring-harness-upgrade/
This link shows a Universal Wiring clean up. I know it is not your "saildrive motor" but it covers the basics of electrical connections between the batteries, the alternator and your specific question the starter. The basics here also apply to your electrical connections. The issue is many of our boats were wired in a poor manner. MaineSail explains the problems and suggests upgrades that make your system safer to operate.
I rewired my electrical system using these techniques and placed a 150 amp fuse between the battery and the starter motor. FSSssssstttt. Gone in the second it took to turn the switch. Replaced with a 300 amp fuse on the large gauge wire as indicated. Starter turns over every time. Motor starts on request. And no FSSsssssttt.
I recall there may be a thread about the starter motor and fusing on the SBO forum.
I encourage you to become knowledgable about this subject not just follow blindly mine or anyones suggestions. You need to be positive about what you are going to do with your electrical circuts. If in doubt hire a Marine Electrician to help you.
Be safe and Good luck.
I'm not shy around electricity. Cabling is like 80% of my job. It looks to be 4awg wiring for all the main components (batteries, batt switch, charger, starter). I've typically used 60-80 amp fuses with 4 awg for car stereos but who knows if a previous owner knew the correct size to begin with. I'm confident 4awg is plenty for this application. The starter looks to be a typical automotive starter. I can't imagine it drawing much when used with this tiny engine. A diesel will definitely draw more because of the higher compression. This is a two stroke.
Nice! Not even an inch of fuse to fry!
If it is 4 ga, then a 160 amp fuse is needed. If that blows, go one size higher.
http://www.genuinedealz.com/voltage-drop-calculator
http://www.genuinedealz.com/voltage-drop-calculator
There is only about 6' of cabling from the battery to the starter. I'm seeing 80 amps for 4awg wire on most sites
BlueSea recommends 70-100A depending on wire type
The starter isn't likely to be anywhere close to max capacity of the wiring. I've seen what 30A@120v can do. I also have a welder rated at 125A can can penetrate upto 3/8" steel. No point in putting a larger fuse than needed. Anyways, what I'm interested in finding out is the power draw of the starter. I can meter most other components to find this info
BlueSea recommends 70-100A depending on wire type
The starter isn't likely to be anywhere close to max capacity of the wiring. I've seen what 30A@120v can do. I also have a welder rated at 125A can can penetrate upto 3/8" steel. No point in putting a larger fuse than needed. Anyways, what I'm interested in finding out is the power draw of the starter. I can meter most other components to find this info
Use a clamp on meter. Also, somewhere on the MarineHowTo.com site, RC has an article on this subject. I did a quick search and couldn't locate it. Short on time right now.
@dlochner MaineSail has experimented with it. He has a video showing the momentary surge and why the type of fuse you pick is important.
Here is reference to the info from MaineSail's site.
Here is reference to the info from MaineSail's site.
Start battery Energy Use?
A battery used for starting an engine is using very little stored energy to do this job. It is a very short duration but also high amperage. Most engines will use considerably less than 0.5Ah to start. This is due to the cranking duration, loaded starter to unloaded starter, averaging 0.75 seconds to about 1.5 seconds (averages measured over 70+ marine diesel engines using the Midtronics EXP-1000HD). This means your previously full start battery will still be at about 99%+ SOC after you’ve started the engine. A 99% SOC battery does not really require immediate charging or priority charging and has many, many, many more starts left in it before any charging would even become necessary.
In the image above we have a 44HP diesels cranking diagnostics:
Averaged cranking voltage = 12.04V
Averaged cranking Amps = 286A
Loaded to Unloaded Cranking Duration = 0.765 Seconds
But there is another where he is actually measuring the surge.A battery used for starting an engine is using very little stored energy to do this job. It is a very short duration but also high amperage. Most engines will use considerably less than 0.5Ah to start. This is due to the cranking duration, loaded starter to unloaded starter, averaging 0.75 seconds to about 1.5 seconds (averages measured over 70+ marine diesel engines using the Midtronics EXP-1000HD). This means your previously full start battery will still be at about 99%+ SOC after you’ve started the engine. A 99% SOC battery does not really require immediate charging or priority charging and has many, many, many more starts left in it before any charging would even become necessary.
In the image above we have a 44HP diesels cranking diagnostics:
Averaged cranking voltage = 12.04V
Averaged cranking Amps = 286A
Loaded to Unloaded Cranking Duration = 0.765 Seconds
That table tells you how large the wire needs to be for “X” amps of current to have a voltage drop of 3 (or 10) %, with “Y” feet of wire. It really doesn’t tell you what size fuse to use. Normally you want the fuse to protect the wire from a dead short, so use this table: https://www.bluesea.com/support/reference/529/Allowable_Amperage_in_Conductors_-_Wire_Sizing_Chart, I think.There is only about 6' of cabling from the battery to the starter. I'm seeing 80 amps for 4awg wire on most sites
BlueSea recommends 70-100A depending on wire type
The starter isn't likely to be anywhere close to max capacity of the wiring. I've seen what 30A@120v can do. I also have a welder rated at 125A can can penetrate upto 3/8" steel. No point in putting a larger fuse than needed. Anyways, what I'm interested in finding out is the power draw of the starter. I can meter most other components to find this info
@marchem is correct, it is not the voltage drop to fuse for, but the capacity of the wire to handle amperage regardless of length.There is only about 6' of cabling from the battery to the starter. I'm seeing 80 amps for 4awg wire on most sites
BlueSea recommends 70-100A depending on wire type
The starter isn't likely to be anywhere close to max capacity of the wiring. I've seen what 30A@120v can do. I also have a welder rated at 125A can can penetrate upto 3/8" steel. No point in putting a larger fuse than needed. Anyways, what I'm interested in finding out is the power draw of the starter. I can meter most other components to find this info
When considering voltage drop the calculation is based on the "round trip" of the circuit. If the starter is 6 feet from the battery, the round trip is 12'. Often boat builders use the smallest wire they can get away with to cut down on build costs. Sizing up to a 2 ga might yield better starting.
Actually, there is.
The larger the fuse the lower the voltage drop at the fuse.
As Dave L said earlier, 4AWG wire has an ampacity of 160amps when out side of the engine room. So you could use a 150 or 160 amp fuse.
If you decide to use a 70 or 80 amp fuse, you will have more voltage drop at the fuse. Maybe its negligible in your instance but using the largest fuse that protects the wire does have an advantage.
Here is a good article that explains fusing, terminations and voltage drop:
https://marinehowto.com/fusing-termination-voltage-drop/
Years ago measured the starter current on my Johnson 9.9hp outboard.
38 amps constant cranking, I did not attempt to measure the peak.
38 amps constant cranking, I did not attempt to measure the peak.
Most fuses have a current curve that allows a much higher peak current than the fuse rating (size) for a short time. Fuses sold by Blue Seas Systems provide this time/current curve in their catalog. This property of the fuse will usually accommodate a starting motor's initial cranking demand.
I think it will depend on how close to the peak the draw is. If the fuse is getting maxed out on a regular basis, its life might be shortened. So if the peak is 140 amps and the fuse can sustain a 140 surge, it wouldn't blow, but its life might well be shorter than if the fuse could sustain a 200 amp surge. Like the old fashioned incandescent light bulbs, the more they cycled on and off, the shorter the life span.